1. Technical Field
The present invention relates to a surface acoustic wave device using the upper limit mode of the stop band of Rayleigh surface acoustic waves.
2. Related Art
Surface acoustic wave devices such as SAW resonators or SAW filters, features higher frequencies, smaller sizes, and mass production capability, having widely been used in the field of communications. In particular, surface acoustic wave devices using a quartz substrate such as ST-cut quartz substrates shows high temperature stability featured by a quartz crystal so as to attain higher accuracy. As the use of portable communications apparatuses spread in recent years, those surface acoustic wave devices are further required to achieve higher frequencies and smaller sizes as well as higher temperature stability and thereby higher accuracy.
It is known that two frequency solutions called “stop band” can be calculated with respect to a Rayleigh surface acoustic wave excited by an interdigital transducer (IDT) electrode provided on a piezoelectric substrate made of a crystal or the like. Either of these frequency solutions, that is, the lower frequency (lower limit mode) and the higher frequency (upper limit mode) is used in excitation. It is known that when an ST-cut quartz substrate includes a single-type IDT electrode having two electrode fingers in one wavelength of a surface acoustic wave, surface acoustic waves are excited in the lower limit mode of the stop band. In the meantime, as shown in the Technical Report of the Institute of Electronics, Information and Communication Engineers (IEICE), US99-20 (1999-06), pp. 37-42 (FIG. 4), when comparing the lower and upper limit modes, the upper limit mode shows a smaller absolute value of the second-order temperature coefficient of the frequency temperature characteristic (frequency variation characteristic with the temperature). The upper limit mode also shows a smaller variation (smaller increase or decrease) in the absolute value of the second-order temperature coefficient when the thickness of the IDT electrode is increased. Therefore, it is understood that a better frequency temperature characteristic is exhibited in the upper limit mode and that the upper limit mode is more suitable to obtaining higher frequencies. However, a single-type IDT electrode on the ST-cut quartz substrate can excite no surface acoustic wave in the upper limit mode.
Thus, as means for exciting surface acoustic waves in the upper limit mode of the stop band, there has been proposed a surface acoustic wave device including a reflection/inversion type IDT electrode as shown in JP-A-2002-100959 (FIG. 13). FIGS. 11A and 11B of the subject application show the configuration of a surface acoustic wave device including a reflection/inversion type IDT electrode; FIG. 11A is a schematic plan view, and FIG. 11B is a schematic cross sectional view taken along line C-C of FIG. 11A. In a reflection/inversion type IDT electrode 51, electrodes 52 and 53 are configured with its electrode fingers 61, 62, and 63 disposed as if to be engaged to each other. According to this configuration, three electrode fingers 61, 62, and 63 are provided in one wavelength λ of a surface acoustic wave, with the electrode fingers 61,62 and 63 driven in opposite phases.
Further, in order to improve the frequency temperature characteristic when using an ST-cut quartz substrate in a surface acoustic wave device, it is known to use an in-plane rotation ST-cut quartz substrate as taught in “Temperature Stability of Surface Acoustic Wave Resonators on In-Plane Rotated 33° Y-Cut Quartz,” JJAP Vol. 42 (2003), pp. 3136-3138. According to this document, the frequency variation is about 59 ppm when Euler angles are (0°, 123°, 43.4°), the second-order temperature coefficient in the lower limit mode of the stop band is −1.4×10−8 (1/° C.2), and the temperature is in the range of −40° C. to 90° C.
However, the reflection/inversion type IDT electrode includes three electrode fingers in one wavelength. Therefore, in order for a surface acoustic wave device using an IDT electrode of such a type to achieve higher frequencies, it is necessary to make the width of the IDT electrode even smaller than that of a single-type IDT electrode, which commonly includes two electrode fingers in one wavelength of a surface acoustic wave. This places a burden on the manufacturing process, making it difficult for a surface acoustic wave device using a reflection/inversion type IDT electrode to achieve higher frequencies.
Further, even when using an in-plane rotation ST-cut quartz substrate in a surface acoustic wave device in order to improve the frequency temperature characteristic to achieve higher accuracy, the frequency variation is about 59 ppm at best in the temperature range of −40° C. to 90° C.